The science says: If you are able to hit 15 repetitions of any given exercise at a certain weight, then it is too light for you. Why? After 15 reps, it starts to become more of an endurance exercise. If your goal is endurance, then keep the weight. If your goal is muscular size and strength, increase the weight.
Maybe when you increase the weight, you can only do 10 or even 8. That is okay! Keep using that same weight week after week until it gets to 15 and then you can do it all over again and increase the weight a second time. Do this enough times and over the course of a few short months, you will get much strongner.
Pro Tip: Any muscle you truly want to build quickly and efficiently, it is MANDATORY to train that muscle twice a week. Just make sure it gets enough recovery.